Process for the oxidation of mercaptides



Dec. 1 3, 1938. D. l.. YABROFF ET Al.

PROCESS -FOR THE OXIDATON OF MERCAPTIDES Filed Aug. 19, 1936 Marra/#fabs Potootod Deo. 13, 193s UNITED STATES PROCESS F03 THE OXIDATION MERCAPTIDES- of Delaware Application August 19, 1936, Serial No. 96,708

scialms.

'I'his invention relates to the electrolytic oxldation of mercaptides to disulfldes in aqueous alkaline reacting solutions.

In the desulfurization of hydrocarbon oils large quantities Aof alkali hydroxides or other alkaline reacting hydroxides are used for the extraction of mercaptans. obnoxious mercaptide solutions as-well as the high consumption of treating chemicals make regen- 0 eration of the hydroxidoo without the use of additional chemicals very desirable.

It is the purpose of this invention to proapparatus which in practice has given good resuits. -Figure 2 shows a multistage apparatus consisting essentially of several treating units of the type shown in Figure 1. Referring to Figure 1, an aqueous solution of mercaptides isV conveyed by pump I in line 2 into a cylindrical treating vessel 3. Rotatable shaft I is axially dis- .posed in vessel 3, resting in insulated bearing 5 on the bottom plate of vessel 3. Attached to the shaft 4 by means of brackets 6 is concentrically arranged a perforated rotor 1. Shaft 4 extends upward through insulated stuiilng box on top Iao of vessel 3 and is connected by means of an electrical insulator 8 to motor 3 which is preferably ofthe variable speed type. Vent 2l on top of vessel 3 serves for the escape of hydrogen produced on the cathode during the treatment.

'Ihe apparatus is supplied with a direct electric current from a 2 to 4 volt generator II. The current passes through wire I2, ammeter 23, and variable resistance 22, which Aserve to measure and to control the current strength. The current then passes through electric brush I0 attached to shaft 4, through shaft 4, bracket 6, rotor 1, through the electrolyte in cell 3, through contact I3 attached tocell 3, through wire I4 back to generator AII. Rotor 1 maybe either anode or cathode. Since it is desirable that the anode has as large a surface as possible for The disposal of the resulting.

(ci. zn-s) reasons hereinafter explained, we generally prefer to use rotor for the anode.l

While the aqueous alkali mercaptide solution passes through vessel 3 a portion of the mercaptide is convertedl to disuldes. The resulting mixture passes through transfer line I6 to settling tank I1, where the disulfides are allowed to separate and rise to the top, to be withdrawn through line I8, while an aqueous hydroxide which is at least partly regenerated, leaves through line I9.

The electrodes, i. e. vessel 3 and rotor 1, may be constructed of any material of good electrical. conductivity, which is resistant to the action of the aqueous hydroxide, and in the case ofthe anode, is also resistant to oxidation. 'I'he fol-- lowing materials have lproven to be satisfactory: gold, silver, the platinum metals, iron, cobalt, nickel, stainless steels and other -ferrousv alloys, graphite and carbon. ALead and lead oxides are easily attacked by aqueous alkali hydroxide but may be used under some circumstances. Stainless steels, and particularly the alloy known as KA 2 -metal, are very satisfactory. They are highly resistant to caustic at temperatures be` low about 100 C., strong, relatively cheap, and are preferentially wetted by aqueous hydroxide, rather than by the disulfldes formed in the process. Graphite and carbon, while often useful, have the advantage of being brittle and tending tocrumble, thereby` contaminating the aque-- ous hydroxide.

Figure 2 shows 3 units of the type described in connection with Figure 1, arranged inv series f with respect to the ow of the aqueous hydroxide, Y which is pumped by pump 25 successively through treating vessels and settlers TI, SI, "12, S2, T3, ISB. Regenerated aqueous hydroxide is withdrawn through line 28 while disulfldes from the several treatments are collected in manifold 21. With respect to the electric current generated by the direct current generator G, ythe treaters TI, T2, and T3 are arranged in parallel. 'In order to enable the regulation of the current densityl through each treater individua1ly,so as to maintain optimum treating conditions in each treaterl in spite of the changing composition of the aqueous hydroxide from treater to treater, each circuit is equipped with individual Variable resistance, RI, R2, and R3,frespectively.

While we have described in the foregoing a particular type of cylindrical electrolytic cell; containing a rotatable cylindrical anode, it shall b e understood that other forms of cells are suitable as well. A type of cell which is frequently -.used in electrolysis, lcomprising a series-of alternating positive-and negative electrode plates in a rectangular cell, has also given satisfactory results.

Our process is applicable to any aqueous solution of mercaptide andk isparticularly useful when applied to the mercaptide solutions obtained in the treatment of petroleum oils with aqueous alkalinel reacting hydroxides. Theterm alkaline reacting hydroxides, as used in this specification, is meant to apply to hydroxides which are capable of extracting mercaptans from their nonaqueous solutions by forming water soluble mercaptides and includes the alkali metal hydroxides, alkali earth hydroxides, and Quaternary ammonium bases. The concentration of the mercaptide solutions, which are susceptible to our treatment, may vary within wide limits, the only requirement being that they be liquid.

Normally solid aqueous alkali hydroxides containing mercaptides and from 2-50% water may be subjected to our process at temperatures suinciently high to liquefy them.

The treatment may be carried out advantageously at temperaturesbetween about and 100 Cyandpreferably between 20 and 60 C. although higher or lower temperatures may be employed. At relatively low temperatures below about 0' C. undesirable side` reactions such as the formation of sulfonic acid tend to proceed at harmful rates; and at high temperatures in excess'of about 100 C. many electrode mate-Y rials, which at lower temperatures` are suitable and resistant to the combined action of caustic and oxygen, are severely attacked and corroded. To avoidl undue foaming, a temperature below the boiling point oi' the alkaline solution is preferably maintained at all times.

Aqueous mercaptide solutions subjected to our process may contain organic substances such as alcohols, alcohol ethers, alkanolamines, etc. which may have been Iadded to the kalkaline reacting hydroxides to act as solutizers for mercaptans in the aqueous hydroxides to aid in the extraction. Such organic substances do not interfere with ourprocess, provided they are less readily oxidized than mercaptides. The. presence of salts of aromatic hydroxy compounds of the type of phenol, cresols, xylenols, however, interiers with the oxidation of mercaptides insofar as these substances are oxidized jointly with the mercaptides top .undesirable compounds which .frequently are of a tarry character. Carboxylic acids such as fatty acids or naphthenic acids or their salts may be oxidized to hydrocarbons and carbon dioxide, the latter being converted to carbonate which cannot be easily reconverted to the hydroxide. Hydrogen sulilde salts are oxidized to thiosulfates and other Oxy-sulfur compounds which use an equivalent amount of caustic.

Since hydrogen sulfide, carboxylic acids and hydroxy aromatic compounds frequently are constituents of petroleum oils, we usually pretreat such oils in a manner to remove them prior to extracting the mercaptans. Such pretreatment may be accomplished, for instance, by fractionally distilling the petroleum oil to eliminate hydrogen sulfide, water washing to remove the low fatty acids, treating with a concentrated aqueous alkali soap solution containing free alkali boxylic acids and/or hydrogen sulfide, and therefore the former are most likely to contaminate the alkaline solutions of mercaptides. We have fouhd'that as long as the molal ratio of hydroxy aromatic compounds to mercaptides in the solution does not exceed about 1, sufficiently high current eiliciencies can be maintained, particularly in the presence of catalysts of a type hereinafter described, to make the process commercially practical.

During the electrolytic treatment of the mercaptide solution, negatively charged mercaptide ions travel to the anode where they are discharged according to, the equation:

Where RS=mercaptide ion E=negative electric charge Instead of forming disulfldes on the anode, the mercaptides may also be converted to sulfonic which are insoluble in aqueous alkaline reacting acids. Whereas', disulfides are neutral compounds hydroxides and are easily separated therefrom by settling, centrifuging or washing with a suitable solvent such as naphtha, sulfonic acids form water soluble sulfonates with many of the aforementioned alkaline reacting hydroxides, and cannot be separated from aqueous alkaline solutions by simple means. If sulfonic acids are produced, the alkaline reacting `hydroxide. is gradually converted to sulfonates, which are not readily reconverted to the hydroxides. Therefore, it is our aim to maintain such conditions as to avoid the oxidation of mercaptides to sulfonates as much as possible.

It appears that the oxidation of mercaptides to sulfonates depends upon the discharge of hydroxyl-ions lon the anode, so that anything that will prevent the discharge of hydroxyl ions will also prevent the formation of sulfonates. Hydroxyl ions have a higher discharge voltage than mercaptides, and, therefore, it has been found desirable to operate in the narrow voltage range, in which mercaptide ions, but not hydroxyl ions are discharged. The minimum voltage at which mercaptide ions are discharged at a practical rate is about 1.5 volts. Under many circumstances, however, the voltage drop across the cell may rise considerably above this minimum, thus promotln'g the discharge of hydroxyl ions; such fluctuations preferably should be avoided. Ihe main factors controlling this over-voltage are anodic current density, concentration and type of mercaptides, catalysts, nature of anode, temperature of treatment and alternating electric potentials superimposed on the direct current.

The higher the anodic current density, i. e. the amperes per unit area of the anode, the higher is also the over-voltage. 'I'his places a definite limit onv the current density, which we found must not exceed about 10 amperes per square decimeter even under the most favorable conditions. Under less favorable conditions, a lower. density must be maintained. 'I'he maximum permissible density must be established experi- 'mentally for each set of conditions by samplingJ the aqueousalkaline reacting hydroxide after passage through the cell and determining analytically the amount of sulfonic acids contained therein. If a substantial amount of sulfonates is found in the outgoing solution, the current density must be reduced.

` In order to maintain a relatively high current at a minimum current density, it is desirable to enlarge the surface of the anode as much as possible, for instance by corrugating t the rotor in Figure 1.

The maximum permissible amount of sulfonate in the hydroxide is entirely a question of eco-v nomical considerations, and cannot be specifled with any degree of accuracy. It appears, however, that if about 10% or more of the mercaptides are converted per passto sulfonic acids, the hydroxide will soon be rendered resistant to practical regeneration. v A

'I'he effect of the current density on the formation of sulfonates is wellillustrated by the following figures in which .1' normal solution of butyl mercaptan in a 4 normal sodium hydroxide solution was electrolyzed;

Sulfonic acid in percent of Current density amp/sq. decimctexl total merca? Percent l m mercaptides.

We found that the resistance toward conversion to sulfonates'V varies considerably with different As a general rule,tertiary mercaptides are more easily converted to sulfonates than secondary or primary mercaptides, and the resistance'increases with increasing length of the carbon chain.

'I'he concentration of -mercaptides is of importance, since fora given current density,`the

over-voltage increases with decreasing concentration. Hence at low concentrations of mercaptides,there is a relatively greater danger of forming sulfonates, and currents of relatively low current densities must be used. Since it is desirable to use high current densities wherever possible to reduce the time of oxidation andthe size of treating equipment to a minimum, we may begin treating a mercaptide solution with a current of relatively -high density. As the mercaptide concentration decreases, due to the conversion of mercaptides to disulfides, we then reduce the current densi-ty just enough to avoid substantial formation of sulfonatesl This method of treating may be carried out in a single cell by gradually increasing the resistance in the electric circuit by meansI of an adjustable rheostat; or in a series of cells of the type shown in the aforedescribed Figure 2, a current density being maintained in 1 each cell which is lower than that of thevpreceding cell, the difference vbeing -sufllcient to counteract theelect of decreasing mercaptide concentration on the formation of sulfonates.

In order to avoid local depletion of mercaptides from'the immediate vicinity of the anode, which may lead to the liberation of free oxygen, we l usually keep the solution in a state of high turbulence. This has the additional advantage of immediately removing disuldes formed in .the

process from the anode, thereby reducing the danger of rendering a portion of the anode inactive due to accumulation of disuldes thereon.

A number of anions capable of forming water soluble salts with the aqueous hydroxide were found'to be active as catalysts in the matter of lowering the over-voltage required to discharge mercaptide ions. Anions of acids containing elementsof the third group of the periodic system, sixth group in the 4 and 6 valent state and seventh group in the 1 and 5'va1ent state are suitable for this purpose. Particularly useful are the following: `borate, sulflte, sulfate, selenite, selen tide oxidized age.

nate, teuunte, tenuate, chromate, fluoride, chiorate, bromide, bromate, iodine and iodate ions.

While the effective concentration of these catalyst ions may vary over a considerable range, an y average of about .005 mol per liter was found practical in most instances.

Certain of these catalysts, such as the chromate and selenite ions catalyze the discharge of mercaptides in preference to that of other oxidizable compounds that may be contained in the aqueous alkaline reacting hydroxide, for instance, compounds of the type-of cresols. Thus if it should be desired to convert mercaptides to disulfides'in spite of the presence of oxidizable impurities other than mercaptides which tend to interfere with our process, this can be done successfully by adding one or several of the above catalyststo the mercaptide solution. The following data illustrate the effect of a typical catalyst on the formation of sulfonates: A .1 normal solution of ethyl mercaptide in a 4 normal sodium hydroxide solution was oxidized with and without borate as catalyst with the following results:

Sulfbnaies in per cent of total mercaptide oxidized Catalyst Current density ampJsq. decimeter s .005 moll BO; por

iter

A low amperage alternating current of not more than 100 cycles having a density lower than that of the direct current, superimposed on the direct current also greatly lessens the over-volt- For instance, when a 60 cycle alternating current of .2 ampere was added to a direct current of 4.0 amperes passing through a mercaptide solution, the amount of oxygen formed was reduced to about one-half.

We claim as our invention: I

1. In a regenerative process of extracting mercaptans fromsour hydrocarbon oils containing same with an aqueous alkali hydroxide solution capable of extracting mercaptans, thereby forming water-soluble mercaptides, the improvement f comprising treating a sour hydrocarbon oil with said hydroxide solution, whereby a spent alkaline solution of mercaptides is formed, separating the treated hydrocarbon oil from the spent solution; passing a direct electric current of suilicient voltage between electrodes through the solution to oxidize mercaptide ions to water-insoluble disuldes, while maintaining an anodic 'current density below l0 amps/dm. and below that at which a substantial portion of the mercaptides Ais converted to sulfonates, and separating the disuliides from the electrolyzed aqueous solution.

2. The process of claim l in which the aqueous solution Icontains an effective quantity oi' a catalyst anion'capable of reducing over-voltage required to discharge mercaptide ions, said catalyst anion being selected from the group of borate, suliite, sulfate, selenite, selenate, tellurite, tellurate, chromate, fluoride, chlorate, bromide, brornate, iodide, and iodate ions.

3. The process of claim 1in which an alternating current of less than 100 cycles having a density below that of the direct current is superimposedon the direct current,

4. The process of claim 1in which the aqueous solution is kept in a turbulent state to prevent local depletion of mercaptide ions from the immediate vicinity of the anode.

5. In a regenerative process of extracting mercaptans from sour hydrocarbon oils containing same with an aqueous alkali hydroxide solution capable of extracting mercaptans, thereby forming water-soluble mercaptides, the improvement comprising treating a sour hydrocarbon oil with said hydroxide solution, whereby a spent alkaline solution of mercaptides is formed, separating the treated hydrocarbon oil from the'spent solution, and flowing the latter through at least two separate serially arranged electrolyzing zones, passing through the solution contained in each zone direct electric current of sufllcient voltage to oxidize mercaptide ions to water-insoluble disuldes, while maintaining in the several zones progressively decreasing anodic current densities in the direction of the flow of the solution, said current densities being lower than those at which a substantial portion of the mercaptides are oxidized to sulfonates and below 10 amps/dm?, and separating the disuldes from the electrolyzed aqueous solution.

6. The. process of claim 5 in which the disulfldes are separated from the solution after each passage through an electrolyzing zone.

7. In a continuous regenerative process of extracting mercaptans vfrom sour hydrocarbon oils containing same with an aqueous alkali hydroxide solution capable oi extracting mercaptans,

thereby forming water soluble mercaptides, the improvement' comprising treating a sour hydrocarbon oil with said hydroxide solution, whereby a spent alkaline solution containing mercaptides is formed, separating the treated hydrocarbon oil from the spent solution, passing a direct electric currentof suiiicient voltage between electrodes through the latter to oxidize mercaptide ions to water .insoluble disuldes while maintaining Aan anodic current density below 10 amps/dm,

regulating same with decreasing content of mer` captides to keep it below that at which a substantial portion of the mercaptides is converted tov v sulfonates, separating the disulfldes from the electrolyzed aqueous solution and returning the latter for Afurther extracting mercaptans from sour hydrocarbon oil.

8. In a continuous regenerative process of extracting mercaptans from a sour hydrocarbon oil containing same and acidic compounds other than mercaptans, with an alkali hydroxide solution capable of extracting mercaptans, thereby forming water soluble mercaptides, the step of pretreating the sour hydrocarbon oil to removethe acidic compounds without effecting sweetening, treating the pretreated oll with said hydroxide solution, whereby a spent alkaline solution containing mercaptides is formed, separating the treated hydrocarbon oil from the spent solution, passing a direct lelectric current of suicient volt-V age between electrodes through the latter to oxidize mercaptide ions to waterinsoluble disuldes while maintaining an anodic current density be` low 10 amps/dm?, regulating same with decreasing content oi' mercaptides to keep it belowthat at which a substantial portion of the mercaptides is converted to sulfonates, separating the di`' sulildes from the electrolyzed aqueous solution and returning the latter for further extracting mercaptans from pretreated sour hydrocarbon oils free from acids other than mercaptans.

DAVID LOUIS YABROFF. JOHN WILKINSON GIVENS. 

